Gas Exchange in Plants
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You should recall the general principles of diffusion and the structure of the mammalian lung, specifically how surface area and membrane thickness influence gas exchange rates.
Why this matters
Plants must balance the need for carbon dioxide with the risk of water loss.
Unlike animals, which primarily exchange oxygen and carbon dioxide, plants require carbon dioxide for photosynthesis while simultaneously managing transpiration. This creates a physiological conflict: the same structures that allow gas entry must also prevent excessive dehydration. Understanding this balance is crucial for grasping how terrestrial life sustains itself, from the microscopic stomata of a leaf to the global carbon cycle. The efficiency of this exchange determines growth rates, crop yields, and the resilience of ecosystems to climate change.
The Fundamental Trade-off
Imagine a building that needs to import raw materials for construction but must also keep its interior humidity controlled to prevent the materials from drying out and crumbling. In plants, the "raw material" is carbon dioxide, and the "humidity control" is the retention of water. The primary site of this exchange is the leaf, specifically through tiny pores called stomata. Each stoma is flanked by two guard cells that can change shape to open or close the pore. When the pore is open, carbon dioxide diffuses in, and oxygen (a byproduct of photosynthesis) diffuses out. However, water vapour also escapes through this same opening. This is not a passive leak; it is a regulated process. The plant faces a constant decision: how much carbon dioxide is worth the cost of water? If the plant closes the stomata to save water, photosynthesis slows down because carbon dioxide becomes limiting. If it keeps them open, it risks wilting. This trade-off is the central theme of plant gas exchange. The structure of the leaf is adapted to minimise this cost. The internal air spaces are large and interconnected, ensuring that carbon dioxide does not have to travel far through the liquid phase of the cell walls. The mesophyll cells are thin and rich in chloroplasts, positioned close to the air spaces. This arrangement reduces the diffusion distance for gases, allowing for rapid exchange even when the stomata are partially closed. The entire system is a compromise between metabolic demand and environmental constraint.
Stomatal Regulation and Guard Cells
The opening and closing of stomata are controlled by the turgor pressure of the guard cells. When the guard cells take up water via osmosis, they swell and bow outward, opening the stomatal pore. When they lose water, they become flaccid and the pore closes. The primary driver of this water movement is the accumulation of potassium ions (\(\mathrm{K^+}\)) inside the guard cells. In the light, blue-light receptors in the guard cells, helped by the fall in leaf carbon dioxide as the mesophyll photosynthesises, switch on potassium uptake. Proton pumps (\(\mathrm{H^+}\)-ATPases) actively pump protons out of the guard cells, creating a negative electrical potential inside. This potential drives the passive influx of potassium ions through specific channels. As potassium concentration rises, the water potential of the guard cells decreases, causing water to enter from the surrounding epidermal cells. This process is energy-dependent and tightly coupled to light conditions. In the dark the proton pumps slow, potassium leaves through outward channels, and the stomata close to conserve water. This diurnal rhythm ensures that gas exchange occurs primarily during the day when photosynthesis is active. The sensitivity of this mechanism to environmental cues, such as humidity and carbon dioxide concentration, allows the plant to respond rapidly to changing conditions. For instance, if the air becomes dry, the water potential gradient between the leaf and the air increases, accelerating transpiration. The plant detects this water loss and closes the stomata to prevent dehydration. This feedback loop is a classic example of physiological homeostasis.
Diffusion Pathways and Resistance
Once carbon dioxide enters the leaf through the stomata, it must diffuse through the intercellular air spaces and then cross the cell walls and membranes to reach the chloroplasts. This journey involves several resistances to diffusion. The first resistance is the stomatal conductance, which is the ease with which gas moves through the open pore. The second is the boundary layer resistance, the thin layer of still air surrounding the leaf surface. In still air, this layer can be thick, slowing down the supply of carbon dioxide. In windy conditions, the boundary layer is thinner, facilitating gas exchange. The third resistance is the internal leaf resistance, which includes the diffusion through the air spaces and the mesophyll cell walls. Because the resistances act in series, they add, and the total conductance is the reciprocal of that sum. This means that the slowest step in the pathway limits the overall rate of gas exchange. If the stomata are closed, the stomatal resistance is high, and the overall conductance is low, regardless of how efficient the internal diffusion is. Conversely, if the stomata are wide open but the internal air spaces are poorly connected, the internal resistance becomes the limiting factor. Understanding these resistances helps in predicting how plants will respond to environmental changes. For example, in a greenhouse, increasing the carbon dioxide concentration can overcome the stomatal limitation, but only up to a point. If the internal diffusion is slow, adding more carbon dioxide outside the leaf will not increase the rate of photosynthesis. This principle is known as the law of limiting factors.
Transpiration and Water Potential
Transpiration is the loss of water vapour from the plant to the atmosphere. It is driven by the difference in water potential between the leaf and the air. Water potential (\(\Psi\)) is a measure of the potential energy of water in a system. Pure water has a water potential of zero. The addition of solutes lowers the water potential, and pressure can raise or lower it depending on the context. In the leaf, the water potential is negative due to the presence of solutes in the cell sap. The air outside the leaf has a very low water potential because it is dry. This gradient drives water from the leaf to the air. The rate of transpiration is influenced by several factors, including light intensity, temperature, humidity, and wind speed. High light intensity opens the stomata, increasing transpiration. High temperature increases the kinetic energy of water molecules, accelerating evaporation. Low humidity increases the water potential gradient, driving more water out. Wind removes the humid air layer around the leaf, maintaining a steep gradient. While transpiration is often seen as a waste of water, it serves important functions. It cools the leaf, preventing overheating. It creates a negative pressure (tension) in the xylem, which helps pull water up from the roots. This cohesion-tension mechanism is essential for the transport of water and minerals in tall trees. The balance between transpiration and water uptake is critical for plant survival. If transpiration exceeds water uptake, the plant wilts. If uptake runs ahead of transpiration, as on humid nights, root pressure can push droplets out at the leaf margins (guttation). The plant regulates this balance through stomatal control, adjusting the rate of water loss to match the availability of water in the soil.
The concept
Gas exchange surfaces
Why exchange surfaces are thin, moist and large.
A common misconception is that stomata close only when water is short. They also close partly when carbon dioxide is high, even if water is plentiful: with more CO2 outside, the leaf can fix the same carbon through narrower pores and lose less water. This is one reason why rising atmospheric CO2 tends to reduce transpiration per unit leaf area. C
Worked example
Problem. A leaf has a stomatal conductance to CO2 of \(g_s = 0.30\ \mathrm{mol\,m^{-2}\,s^{-1}}\) and a boundary-layer conductance of \(g_b = 2.0\ \mathrm{mol\,m^{-2}\,s^{-1}}\), acting in series. The CO2 mole fraction is 400 µmol mol−1 in the air and 300 µmol mol−1 inside the leaf. Find the CO2 uptake rate.
- Series conductances combine like resistances: \(1/g_t = 1/g_s + 1/g_b = 1/0.30 + 1/2.0 = 3.33 + 0.50 = 3.83\ \mathrm{m^2\,s\,mol^{-1}}\).
- So \(g_t = 1/3.83 = 0.261\ \mathrm{mol\,m^{-2}\,s^{-1}}\). The stoma, the smaller conductance, dominates.
- Mole-fraction difference: \(\Delta c = (400 - 300) \times 10^{-6} = 1.0 \times 10^{-4}\ \mathrm{mol\,mol^{-1}}\).
- Uptake: \(A = g_t\,\Delta c = 0.261 \times 1.0 \times 10^{-4} = 2.6 \times 10^{-5}\ \mathrm{mol\,m^{-2}\,s^{-1}}\), that is 26 µmol m−2 s−1, a typical rate for a sunlit leaf.
Units check. With conductance in \(\mathrm{mol\,m^{-2}\,s^{-1}}\) and the driving difference as a mole fraction (\(\mathrm{mol\,mol^{-1}}\), dimensionless), the flux comes out in \(\mathrm{mol\,m^{-2}\,s^{-1}}\).
The cost in water: water vapour moves through the same pores about 1.6 times more easily, so with a vapour mole-fraction difference of 0.02 between leaf and air the leaf loses about \(1.6 \times 0.261 \times 0.02 = 8.3 \times 10^{-3}\ \mathrm{mol\,m^{-2}\,s^{-1}}\) of water, roughly 320 water molecules for every CO2 fixed.
Three quick tests
- What is the primary ion that accumulates in guard cells to cause stomatal opening?
Answer
Potassium ions (\(\mathrm{K^+}\)). - How does an increase in wind speed affect the boundary layer resistance around a leaf?
Answer
It decreases the boundary layer resistance, facilitating gas exchange. - Why do plants close their stomata in the dark?
Answer
To conserve water, as photosynthesis is not active and carbon dioxide is not required.
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Every lecture and every concept in this unit, with prerequisites and threads.
Lecture notes checked line by line by the editor; every number in the worked example was recomputed.
Concepts cited: T-029 · Threads: regulation, systems